Chickens from lines selected for high (HA) and low (LA) antibody response to sheep red blood cell (SRBC) antigen were Used to study the role of genetic factors involved in resistance to Eimeria tenella and Newcastle disease virus (NDV). Routes of administration were intravenous for SRBC antigen and intranasal for NDV. Chicks were exposed to E. tenella either through a natural challenge via the litter or a controlled oral administration. Differences between lines were observed in resistance to cecal coccidiosis, with line HA chicks being more resistant than those from line LA. Results were similar. for both natural and controlled exposures, Differences in resistance to E. tenella were found among alleles for the I alloystern with degree of additivity from the I⁴ allele on resistance depending on the background genome. Chicks from line HA exhibited higher antibody titers to SRBC and lower titers to NDV than did those from line LA. This pattern was the same regardless of whether antigens were given together or alone. Correlations within lines for birds which received both antigens were positive and significant in line HA and not different from zero in line LA. When NDV was given at the time birds received a booster dose of SRBC, antibody titers for NDV and for primary and booster SRBC were higher in. line HA than line LA. Correlations between all titers were positive and significant in both lines. The data suggest that relationships between titers for SRBC and NDV are influenced by both genetic and nongenetic factors. / M.S.